Electric impedance tomography (EIT) is a process, which is known per se, in which the electric impedance between the feed point and the test point can be calculated by feeding an alternating electric current of, e.g., 5 mAeff at 50 kHz into any electrically conductive body, here preferably into the human body, and measuring the surface potentials resulting therefrom at different points of the body. A two-dimensional tomogram of the electric impedance distribution in the body being considered can be determined on the basis of suitable mathematical reconstruction algorithms by gradually rotating the current feed sites around the body while measuring at the same time the surface potentials along a section plane.
Such a tomogram of the impedance distribution of the human body is of interest in medicine because the electric impedance changes both with the air content and the extracellular fluid content in the tissue. It is thus possible to visualize and monitor with this process especially the ventilation, i.e., the ventilation of the lungs, as well as the changes in the end-expiratory lung volume in a regionally resolved manner.
It is known that ventilated lung areas as well as the changes therein over time can be represented by means of EIT.
For example, the clinical advantages that arise from the reopening of previously closed lung areas are described in detail in the patent “METHODS AND APPARATUS FOR DETERMINING ALVEOLAR OPENING AND CLOSING,” EP 1 137 365 B1. However, this known patent is based on the assumption that the opening and closing of alveoli can be determined by means of EIT and respiration pressures can then be set on the basis of these findings such that the alveoli will just remain open at the end of the expiration.
However, there typically are very great inhomogeneities in terms of ventilation in the thoracic cross section plane in intensive care patients with severe lung diseases, in whom alveolar closing is to be suspected, so that it cannot be assumed that all alveoli behave in the same manner in the EIT voxel (“volume pixel”) being considered. Thus, it appears to be more than questionable whether impedance values that are correlated with the alveolar closing and opening to an acceptable extent can be determined with the prior-art process proposed. It would even be possible in the extreme case in boundary areas between open lung areas and lung areas that cannot be opened based on the EIT voxel size that already opened lung areas are over-distended and this over-distension would be misinterpreted as opening of closed lung areas because of the increase in impedance changes that results herefrom.